Bone Conduction Headsets ("Bonephones") Research

Overview

Most
of what we hear is due to sound waves traveling through the air. Sound waves
travel through the outer and middle ear before arriving at the cochlea in
the inner ear. Sound waves can also get to the cochlea through direct vibration
of the bones in the head. This pathway of sound is known as "bone conduction". The
vibrations carry the sound waves to the inner ear, and set up waves in the
cochlea just like the waves that are set up by air conducted sounds.

"Bonephones" are headsets that create vibrations against the head,
in order to transmit sound to a listener via bone conduction. The term "bonephones"
is generally used to describe modern lightweight bone-conduction headsets
that are designed for applied use with auditory displays, rather the traditional
bone-conduction vibrators used in clinical audiology settings. Bonephones
are distinguished from clinical devices by their potential for stereo presentation of sounds, their
small size, comfort, and standardized input jack.

Utility of Bonephones

In comparison to headphones, using bonephones avoids covering the ears of
the listener. This is important if (a) the listener needs to have the ears
unobstructed (to allow them to hear other sounds in the environment), or (b)
to allow them to plug the ears (to prevent hearing damage from loud sounds
in the environment). We have also demonstrated the potential utility of bonephones
for the SWAN audio navigation system.
That study was reported at ICAD 2005. <ICAD
Paper in PDF>

Here are some other advantages of bonephones:

Bonephones retain the same privacy and perceptual constancy that standard
headphones offer.

Bonephones cater to the preferences of visually impaired users who do
not feel comfortable having their ears covered.

Bonephones allow sound presentation to those who have middle and outer
ear disorders, such as aural atresia and microtia.

Research Program

Since bonephones are relatively new, there is a lot of research that needs
to be done to understand how well we hear with them, how effective they are
at presenting different sounds, and their abilities to present stereo and
spatialized ("3D") audio signals. Research in the GT Sonification Lab,
led by Ray Stanley, is addressing
all of these issues.

Audibility Thresholds

We
examined the hearing thresholds for pure tones at a wide range of frequencies,
with (1) the ears open, (2) the ears plugged with foam earplugs, and (3) the
ears open in the presence of background noise. The initial findings were presented
at ICAD 2005. <ICAD
Paper in PDF> The key findings are summarized in the graph to the right.

Spatial Audio

Slight timing and intensity differences are crucial in detecting the location
of a sound source. Spatialized audio presentation relies on the ability to
present different signals to the two ears, in order to mimic these interaural
time and intensity/level differences (ITDs and ILDs). Many have suggested
that stereo separation is not possible using bone conduction. However, we
have shown that it is possible.

We used the Coordinate Response Measure (CRM) to show sensitivity to ILDs
and ITDs. In this task, participants identified a target speech signal lateralized
with ITDs or ILDs among masker speech. The performance on this task improved
as a function of ILDs an ITDs delivered through the bonephones.

The first step in spatialization (3D audio) is lateralization (stereo separation
or panning). We wanted to show that this is possible, and to examine the amount
of lateralization that can be achieved with bonephones.To test this, we had
participants adjust an indicator on a model head to show the lateralization
of a sound source traveling through headphones or bonephones.We found that
participants achieved as much lateralization through bonephones as through
headphones.

HRTFs for Bonephones

As we continue toward developing a true 3D audio impression with bonephones,
we are working on a "Bone Related Transfer Function" or BRTF, that will be
a function we can apply to an HRTF, in order to make sounds intended for headphones
work with bonephones. To begin establishing BRTFs, we used a cancellation
methology to generate a set of bone-to-air shifts that can be used to form
a BRTF. In this task, participants adjusted the phase and amplitude of tones
at a given frequency until cancellation ocurred. One tone was delivered through
headphones and one through bonephones. This work is the main topic of Ray
Stanley's Masters Thesis.

Speech Intelligibility via Bone Conduction

In collaboration with Dr. Andrzej Przekwas and colleagues at CFD Research Corp. in Huntsville Alabama, we are studying speech intelligibility for sounds presented via bone conduction and air conduction.

Dr. Andrzej Przekwas and colleagues at CFD Research Corp. in Huntsville Alabama, for their amazing 3D models of the human head.

This research has been supported, in part, by grants from the US Army. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the funding agencies or sponsors.